Virtual particles and black holes

Why are virtual photons and gravitons allowed to pass outwards through the event horizon of a black hole? Is it that it doesn't really make sense to assign a particular path to a virtual photon? This still isn't satisfying to me, as there is *no* path that allows a real photon to escape.

Further confusing me on this issue, part of the Hawking radiation process is for one of a virtual pair of particles to fall into the black hole and be trapped there. Why is that virtual particle trapped, while a virtual photon is free to escape?

And, if I can slip one more question into this already-crowded post: I've heard, as supposed proof for charge, mass and spin being exceptions to the no-hair rule, the fact that charge, energy and angular momentum wouldn't be conserved if these three characteristics were lost (radiated). This doesn't seem right to me, since I can put a charged object inside a shielded box and not violate conservation of charge. Is the real reason just that they can't be radiated?

(If people would prefer it, I can move these questions (or just the last one) over to the GR board.)

This question concerns the so-called semi-classical approximation, which goes outside gtr to the realm of QFTs, so it can probably stay here.

Graduate students can see Jennie Traschen, An Introduction to Black Hole Evaporation, in Mathematical Methods of Physics, ed. by A. Bytsenko and F. Williams, World Scientific, 2000, for a very nice introduction.

Why are virtual photons and gravitons allowed to pass outwards through the event horizon of a black hole? Is it that it doesn't really make sense to assign a particular path to a virtual photon? This still isn't satisfying to me, as there is *no* path that allows a real photon to escape.

I believe what you are describing doesn't happen. The virtual pair is born just outside the event horizon. One particle falls in and the other doesn't.

I believe what you are describing doesn't happen. The virtual pair is born just outside the event horizon. One particle falls in and the other doesn't.

In the case of Hawking radiation, yes. But if the mass and charge of the black hole are able to exert gravitational and EM forces on outside particles, then virtual gravitons and photons (respectively) must be able to escape, as the force carriers.

The fact that one type of virtual particle is allowed to escape while the other is not, is exactly what is confusing me.

But if the mass and charge of the black hole are able to exert gravitational and EM forces on outside particles, then virtual gravitons and photons (respectively) must be able to escape, as the force carriers.

The force carrying particles don't need to cross the event horizon. The black hole would have been formed from collapsing matter, which would have had a given charge and mass and so emitted virtual gravitons and photons. As the collapse proceeds these will be more and more time dilated, and so the forces we see will be from virtual particles emitted by matter which hasn't yet crossed the event horizon.

The force carrying particles don't need to cross the event horizon. The black hole would have been formed from collapsing matter, which would have had a given charge and mass and so emitted virtual gravitons and photons. As the collapse proceeds these will be more and more time dilated, and so the forces we see will be from virtual particles emitted by matter which hasn't yet crossed the event horizon.

Interesting! But it brings up another issue for me. Let's say that an isolated star collapses to a black hole, so that no other matter falls onto it afterwards. Then you say that any charge or mass we sense (let's talk about just charge, since it is simpler) would, from our far-away Minkowski perspective, come from the star matter as it falls ever-slower toward the event horizon, never quite reaching it. But, as you also say, these virtual photons will be more and more gravitationally redshifted with time. Eventually we will no longer be able to sense them, right? Does that mean that, in the limit of infinite time, an isolated black hole has no (detectable) charge (or even mass)? That can't be right, can it?

Then you say that any charge or mass we sense (let's talk about just charge, since it is simpler) would, from our far-away Minkowski perspective, come from the star matter as it falls ever-slower toward the event horizon, never quite reaching it. But, as you also say, these virtual photons will be more and more gravitationally redshifted with time. Eventually we will no longer be able to sense them, right? Does that mean that, in the limit of infinite time, an isolated black hole has no (detectable) charge (or even mass)? That can't be right, can it?

Well I realise that it seems counterintuitive, but the charge and the mass will indeed stay the same to an outside observer (neglecting Hawking radiation). Quite how this fits with it coming from ever more redshifted virtual particles I'm not sure, but I would guess that it would be possible, but tricky, to demonstrate how this comes out of the mathematics of the situation. It would be tricky because the normal way to think of gravity in this situation is as the curvature of spacetime rather than as a force or as mediated by gravitons.

I believe what you are describing doesn't happen. The virtual pair is born just outside the event horizon. One particle falls in and the other doesn't.

a blackhole is an object having mass. so it shows a presence of some field consider it as a magnetic or a gravitational field. this field produces fluctuations in the form of particle antiparticle pair ON THE EVENTHORIZON . the antiparticle exists for a short period while the particle absorbs some energy and escapes out from the blackholes event horizon. as this energy is in the packets called quanta it can radiate. this is called hawking radiation.my question is except hawking radiation blackholes accreation disc also emits radiation. how can it be possible????

In the case of Hawking radiation, yes. But if the mass and charge of the black hole are able to exert gravitational and EM forces on outside particles, then virtual gravitons and photons (respectively) must be able to escape, as the force carriers.

The fact that one type of virtual particle is allowed to escape while the other is not, is exactly what is confusing me.

Thanks for the reference, Chris Hillman, I'll take a look.

its not actually a virtual particle but a particle antiparticle pair in which the antiparticle exists for a shorter period of time. and blackhole doesnt exert any gravitational or em force outside. the only thing its field creates some fluctuations outside.